Index: Lib/test/test_datetime.py
===================================================================
--- Lib/test/test_datetime.py	(revision 53927)
+++ Lib/test/test_datetime.py	(working copy)
@@ -1425,6 +1425,15 @@
             self.assertRaises(ValueError, self.theclass.utcfromtimestamp,
                               insane)
 
+    def test_negative_float_fromtimestamp(self):
+        # The result is tz-dependent; at least test that this doesn't
+        # fail (like it did before bug 1646728 was fixed).
+        self.theclass.fromtimestamp(-1.05)
+
+    def test_negative_float_utcfromtimestamp(self):
+        d = self.theclass.utcfromtimestamp(-1.05)
+        self.assertEquals(d, self.theclass(1969, 12, 31, 23, 59, 58, 950000))
+
     def test_utcnow(self):
         import time
 
Index: Modules/datetimemodule.c
===================================================================
--- Modules/datetimemodule.c	(revision 53927)
+++ Modules/datetimemodule.c	(working copy)
@@ -3683,6 +3683,12 @@
 		return NULL;
 	fraction = timestamp - (double)timet;
 	us = (int)round_to_long(fraction * 1e6);
+	if (us < 0) {
+		/* Truncation towards zero is not what we wanted
+		   for negative numbers (Python's mod semantics) */
+		timet -= 1;
+		us += 1000000;
+	}
 	/* If timestamp is less than one microsecond smaller than a
 	 * full second, round up. Otherwise, ValueErrors are raised
 	 * for some floats. */